Did you know the U.S. government recommends you send your passport application in a special envelope? Or that losing a lot of weight may trigger the need for a renewal? Would you be surprised to find out that some people's passports are longer than short novels? Here are nine impressive, surprising and vital facts about U.S. passports. You never know when one might apply to you.

Passports are on the rise

In 1996, the U.S. Department of State issued 5.5 million passports. In 2014, that number jumped to 14 million passports. Even when you factor in the population increase, that's a heartening jump in the number of people eager to get out and explore the world.

Protect your passport with ... Tyvek?

Think you can use any old envelope when you're sending in your old passport for renewal? Think again. The Bureau of Consular Affairs strongly recommends you mail your passport application and personal documents using "a secure means of packaging, such as a Tyvek envelope," which will protect against the rough-and-tumble world of postal transit.

You may need to renew sooner than you think

Don't take your passport's expiration date at face value. Some countries have a six-month or three-month passport validity rule that requires your passport to be valid for a certain amount of time after your date of entry.

Your passport has identity theft deterrents

If your passport was issued after August 2007, you've got an e-passport with a small integrated chip in the back cover that stores your passport information and a biometric identifier based on your photograph. To protect passport holders from unknowingly falling victim to high-tech identity theft, there are metallic elements in the cover of the passport, making it impossible for the passport to be digitally "read" until it's physically open.

If you've gained or lost weight, you may need a new passport

If your appearance has changed significantly, you'll need to apply for a new passport. That means if you've lost (or gained) a lot of weight, so much so that you look different than you do in your passport photo, you'll need a new one. The same is true if you've undergone "significant facial surgery or trauma," or if you've added or removed large facial tattoos or piercings.

You must obey photo restrictions

You may not realize it, but nowhere are the fashion stakes as high as in your passport photo. That's because the photo may be rejected -- thus throwing your application into a delayed spiral -- if it doesn't meet certain criteria. The photo must have been taken within six months of your application date, and needs to reflect your current appearance. You must directly face the camera and your expression should be neutral. According to the Bureau of Consular Affairs, "Photos with exaggerated expressions and squinting will not be accepted." And uniforms and "clothing that looks like a uniform" are forbidden as well.

A passport can be novella-length

A standard-issue passport is 28 pages long, and when you're applying or renewing, you can request a longer, 52-page passport at no additional cost. If you need more pages in your existing passport, you can send it in and have additional blank visa pages added in increments of 24 pages, up to a total of 76 pages. A 76-page passport sounds like a great read to us.

Damage can render your passport invalid

Normal wear and tear is forgivable, but if your passport has been significantly damaged, it's time for a new one. Water damage, significant tearing (especially on the book cover or the page with your personal data and photo), unofficial marking on the data page (keep kids with crayons well clear) and torn out visa pages are among the types of damage that will likely mean you'll need to apply for a replacement.